Adrian Mellon's pummeling and death at the hands of Pennywise and how his lover was forced to watch.
The whole opening scene is just heartbreaking. Beyond the Wretched Hive that is Derry, Adrian and Don were two genuine Nice Guys who had no-ill intent, they just wanted to live peacefully in a relationship, but because of how crapsack of a town Derry is, that all gets completely thrown out of the door.
A little girl named Victoria is lured in by Pennywise and devoured by him. What makes it especially sad is we get to know her briefly as a sweet young girl who feels conscious over a port-wine stain birthmark on her face, and while she immediately knows Pennywise is scary, he plays to her sympathy as 'having no friends' himself and promises to remove the mark, resulting in her trusting him, and then her horrible death.
Worse, Victoria was the same girl that Adrian Mellon gave the stuffed animal to, and like her, he became one of Pennywise's victims. She literally loses the one person who showed her some kindness.
Stan's fate hits harder with the added flashback scene of a young Beverly telling the Losers' Club that she saw older versions of all of them, back in the cistern. When Stan asks what he looks like, Bev just replies "you look the same, but taller".
We get a glimpse into the kids' adult lives. While their lives are by no means perfect, they seem to be doing better than how they were as kids, but then we get to Beverly. You think her life would be better off after escaping her abusive father and a town that slut-shamed her, but it's not. The cycle starts up again and she ended up trapped with an extremely possessive husband who beats her. On top of that, she suffered chronic nightmares due to being exposed to the Deadlights. Fortunately, things get better for her after they defeat Pennywise, and she now has a loving husband in Ben.
A flashback shows young Mike talking about how he wants to move to Florida when he grows up, and the smile on his face is just heartbreaking. He had hopes and dreams, too, but he gave them all up to stay in Derry and keep an eye on IT while his friends all moved away and forgot about him.
The montage that plays during Stan's first monologue from his bar mitzvah, showing the young Losers separated and at their lowest points: Beverly covering her drunken father with a blanket, Eddie going back to the Neibolt house to retrieve the placebos he threw away, Ben walking away crying as Bill and Beverly have their moment from the end of the first movie, Mike enduring whispers from the other townspeople implicating him in his parents' death, and Richie secretly carving his love for Eddie into the kissing bridge after enduring a homophobic attack. All while Stan rants in voiceover about how much Growing Up Sucks.
Stan: The things we wish we could leave behind, the whispers we wish we could silence, the nightmares we most want to wake up from, the memories we wish we could change, the secrets we feel like we have to keep...are the hardest to walk away from.
The haunting, heartbreaking shot of Georgie's arms reaching out from the storm drain towards an adult Bill, who looks on with absolute emptiness.
Dean, a little boy who reminds Bill of Georgie, is lured inside a funhouse filled with mirrors with Bill in close pursuit. Eventually, Dean gets trapped in a glass box with Pennywise on one side and Bill on the other. While Bill desperately struggles to break down the glass and save the little boy, Pennywise draws out every last ounce of suspense and anticipation until he gets bored and goes right for the kill. The agony in McAvoy's performance is palpable.
Worse is that Dean was also Richie's fan (turned ex-fan) from the Chinese restaurant that Richie had mistaken for Pennywise in disguise, had lashed out at him to leave him and his friends alone, until he realized that he was actually a normal kid, and even turned down Richie's offer to take a selfie with him as a way to take back what he did. Meaning that, after Pennywise killed Dean in front of Bill, Richie has really lost a young fanboy who was not only accidentally scared off by Richie, but is now dead and is forever unable to make amends with him for the incident back at the restaurant.
Pennywise tormenting Young Ben with an image of Bev that calls him fat, ugly, and disgusting.
The realization that Bill has blamed himself for Georgie's death for 27 years, as he faked being sick since he didn't want to play outside that day.
Bill's vision in the basement only compounds this:
Bill accidentally snaps at Eddie after the latter freezes up in Neibolt's kitchen - an action that almost got Richie killed. A tearful Eddie doesn't even fight back; instead, he apologizes and asks Bill not to be mad at him, almost like a child who has disappointed their parents for the first time. The look of instant regret on Bill's face just seals the deal.
Bill: He could've died! Is that what you want? He could've died!
Eddie: (pressing himself into the wall) No, no! Please ... please don't be mad at me, Billy.
(Bill's eyes widen as he realizes his mistake)
Bill: (softer) That's what IT wants. For you to feel fear. D-d-don't let it, okay?
(Eddie nods tearfully, still unable to look Bill in the eye)
Eddie, mortally wounded by Pennywise, dies with his friends by his side after he proved once and for all that he could be the bravest of them all. Even worse, Richie was in love with Eddie in this continuity, but never had the courage to tell him before it was too late.
Richie's reaction is just devastating. When Beverly tearfully tells him that Eddie is gone, he starts to hug Eddie's body, clearly in denial. When the others begin dragging him away, he refuses to leave Eddie's corpse behind and repeatedly says that he can still be saved. As the Neibolt house collapses upon their escape, he is screaming the entire time and has to be physically held back by both Ben and Mike to stop him from running back in. The anguish in Hader's performance is very evident.
Beverly: (gently) Honey? (touches his shoulder) Honey ... I think he's dead.
Richie: (disbelievingly) What? What, no. He's okay. He's okay, you guys, come on, we gotta help him.
(the house starts to crumble around them)
(Ben and Bill pull him away, although both are visibly upset as well)
Richie: (starts to cradle Eddie's corpse lovingly) NO! No, you guys, come on, we can still help him, please! WE CAN STILL HELP HIM!
It takes two grown men to drag Richie Tozier out of Neibolt. Let that sink in.
When Neibolt is fully destroyed, there's a blink-and-you'll-miss-it shot of Richie simply falling to his knees in despair.
Richie: EDDIE! EDDIE!!
Eddie's death scene is drawn out for a while, but the moment where he first gets impaled is particularly brutal as he's so caught up in his excitement over saving Richie that he turns his back on the monster without checking to see if it's really dead. He's also sitting on top of Richie as it happens, which means Richie quite literally gets showered in Eddie's blood. The sad, scared way they say each other's names and the fact that Richie can only watch in horror as Eddie is dragged away from him doesn't help much, either.
Richie quietly cleaning himself off in the quarry when he sees Eddie's blood on his glasses. He starts to cry for Eddie, which is followed by Bill, Ben, Bev, and Mike hugging him in a manner similar to the first film when the Losers consoled Bill after he found Georgies raincoat.
Although the others are mourning as well, Richie's grief is visibly on a whole different level, seeing as he has been in love with Eddie ever since they were kids.
In a flashback, Richie actually has a good time playing an arcade game with a cousin of Henry Bowers he had feelings for, only to then be turned against and accused of being a "fairy."
Made even sadder is that it didnt even come off as him hitting on Henrys cousin at all. He was just sheepishly asking him if he wanted to play again, almost as if he didnt have friends before this, but the cousin turns around and not only embarrasses him in front of a group of kids but also made him Henrys personal rival for no other reason other than he felt like it.
In some ways, IT's death. Bill Skarsgard had done such a phenomenal job with his take on the Dancing Clown, it's almost hard to watch him go.
The background music as IT dies off is both somber and tense, giving a feeling of absolute despair. Even with this version of Pennywise being far more despicable than his previous incarnations in media, he was still largely seen as a Love to Hate character and pretty much stole the show in every scene he appeared. His fate is very, very richly deserved after all the shit he's pulled, but given his entertainment value and how amazingly well Skarsgard did, it can feel quite bittersweet.
After IT is defeated, the surviving Losers each receive a letter from Stan, explaining that he committed suicide because he was too scared to return to Derry and knew they would have a better chance without him. Bill visibly tears up while reading it, and can only go so far before he breaks down.